1. The earliest stages in development of the Hexactinellid spicule are at present unknown, but there exist reasons for supposing that it originates as a granule enclosed by a spherical syncytium in which cell-outlines are absent, and that the six rays of the hexact grow out from this granule.
2. The earliest stage of development yet discovered is the small hexact, the rays of which do not extend to the periphery of the enveloping syncytium.
3. The rays of the hexact elongate, causing the spherical syncytium at first "to present a more or less octahedral shape, with somewhat concave surfaces and with rounded corners" (Ijima).
4. The rays at length extend beyond the spherical contour of the syncytium (the scleroblast mass), the scleroplasm of which, however, persistently adheres to the rays as a thin film, which occasionally includes nuclei.
5. The peripheral growth of the megasclere type of spicule causes the spherical syncytium enveloping the point of junction of the six rays to dwindle and finally to disappear on account of the distension involved. The whole of the microsclere, with the exception of the terminals, remains permanently enveloped by the spherical syncytium.
6. All tetractinellid and monactinellid spicules originate as granules contained within single cells. In a few instances the spicule arises from several granules within the cell and then consists of separate parts (dragmata, for example).
7. All growth is accretionary. There is no well-authenticated instance of a siliceous sponge-spicule being formed by the fusion of at-first-separate parts--such as occurs in calcareous sponges, for example.
8. With the exception of very large spicules, the nucleus of the distended scleroblast remains single throughout growth.
9. It is a significant fact that spicule formation proceeds on very different lines in each of the three great groups of sponges--the Tetraxonida, Triaxonida, and Calcarea.
Memoirs: Studies in Spicule Formation